1. Field
Embodiments of the present disclosure relate to an induction heating apparatus with an inverter.
2. Description of the Related Art
In comparison with conventional gas burners and oil burners having been used as heaters, the induction heating apparatus has short heating time which leads to less possibility for deformation by heat, and causes self-heating which leads to high efficiency, thereby enabling an entire system to be compact in size and light in weight. Modern high-frequency induction heating apparatuses have a trend to be implemented digitally taking into account protection of switching devices and improved reliability and accuracy. Recent development of power semiconductors, such as Insulated Gate Bipolar mode Transistors (IGBTs), Integrated Gate-Commutated Thyristors (IGCTs), Silicon Controlled Rectifiers (SCRs), Metal-Oxide Semiconductor Field Effect Transistors (MOS-FETs) has enabled resonant converters and inverters in need of high frequency switching to be designed.
The induction heating apparatus is largely divided into two different types. First, there is an induction heating apparatus with a voltage source inverter. The voltage source inverter type induction heating apparatus regulates an output by changing a switching frequency and performs switching at a higher frequency than a desired resonance frequency in a way to draw reactive power from within a resonance circuit. This provides an advantage of decreased reactive power due to the phase delay of a current with respect to a supply voltage and attainment of zero-voltage switching in an automatic turning on phase by using load resonance and switching the load at a frequency higher than the resonance frequency.
However, power loss increases as the switching frequency becomes higher. Accordingly, to reduce the power loss, a circuit, instead of an auxiliary circuit is required or a switching scheme, instead of the zero-voltage switching scheme, for reducing stress on the power semiconductor devices is required.
In contrast, the second type induction heating apparatus uses a high-frequency half-bridge inverter that has advantages of attaining the zero-voltage switching and having low voltage power semiconductors, by nature of the half-bridge scheme, thereby being easily expandable and thus having been widely used in induction heating systems. With the high-frequency half-bridge type inverter, fast heating may be achieved with high reliability, easy control over outputs and temperature, and less pollutants resulting from the heating procedure, thereby securing better heating efficiency, less heat loss and more clean surroundings.
Such an induction heating apparatus of a related art may include an RLC load block having e.g., inductance L from a coil for induction heating, resistance R of an object to be heated, capacitance C of a power factor compensative condenser, and main technologies of the heating induction apparatus are to find a resonance frequency to make impedance and admittance of the RLC load block minimum and thus to supply power to the load at an optimum power efficiency. In other words, it is an important factor that a resonance state is maintained even if the object to be heated (e.g., a pot, a vessel, etc.) has been moved due to a manipulation by a user at the beginning of driving the induction heating apparatus or during heating.
To do this, the induction heating apparatus of the related art measures RLC of a fixed load to select a suitable resonance frequency and performs non-changeable switching for the inverter. That is, since the switching signal sent to the inverter has a fixed resonance frequency, it is difficult to actively react to a situation changed with a change in load state and thus heating efficiency may degrade significantly. In other words, if there is a change in the RCL value due to various factors, resonance frequency deviation may occur and thus induction heating may not work at a maximum efficiency.
In addition, even a change in electrical characteristics due to the load in the induction heating apparatus may require a phase difference between voltage and current applied to a coil to be increased for robust current control, and in this case a switch turn-off current may increase, thereby increasing switching loss.